Intramolecular 1,6-Addition to 2-Pyridones. Mechanism and Synthetic

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Intramolecular 1,6-Addition to 2-Pyridones. Mechanism and Synthetic Scope. Timothy Gallagher,*,† Ian Derrick,‡ Patrick M. Durkin,† Claire A. Haseler,† Christoph Hirschh€auser,† and Pietro Magrone† † ‡

School of Chemistry, University of Bristol, Bristol, BS8 1TS, United Kingdom, and AstraZeneca, Process Research and Development, Avlon Works, Bristol, BS10 7ZE, United Kingdom [email protected] Received March 18, 2010

The scope and limitations of the intramolecular 1,6-addition of an enolate to a 2-pyridone moiety, a reaction that has found application in the synthesis of the lupin alkaloids, have been probed. This nucleophilic addition process has been shown to be reversible and favored in the case of (less stabilized) amide and lactam enolates, which readily form five- and six-membered bi-/tricyclic products. Alternative enolates (ketone, ester, thiolactam) and a variety of different acceptors (isoquinolinone, pyrimidinone, pyrazinone, pyridopyrazinone) have been evaluated, and a range of competing side reactions have been identified and characterized using various techniques, including in situ IR.

Pyridones and pyridone-like heterocycles are key subunits in many biologically important molecules, so the development of novel methodologies to access this class of compounds is of value. Our interest has focused on the use of pyridones as electrophiles (Michael acceptor), and there have been very few examples of nucleophilic attack at the 6-position of 2-pyridones reported. In 1983, Tohda1 described the addition of amine or enolate nucleophiles to highly activated 3,5-dinitro-2-pyridones. More recently,

Hiroya2 and co-workers have employed a carboxy ester to direct and control the Lewis acid catalyzed reaction of an ester-based silyl enol ether with 2-pyridones. Hiroya demonstrated a significant change of selectivity by repositioning the activating ester moiety; while 5-carboxy ester-2-pyridones were attacked predominantly at C(6), the corresponding 3-carboxy ester isomers reacted mainly at the 4-position. Given this background, it is perhaps more appropriate to rationalize these profiles as 1,4-additions with respect to the conjugated ester or nitro substituent rather than to focus on

(1) Tohda, Y.; Ariga, M.; Kawashima, T.; Matsumura, E. Chem. Lett. 1983, 12, 715–718.

(2) Hiroya, K.; Kawamoto, K.; Inamoto, K.; Sakamoto, T.; Doi, T. Tetrahedron Lett. 2009, 50, 2115–2118.

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DOI: 10.1021/jo100514a r 2010 American Chemical Society

Gallagher et al.

the pyridone carbonyl as the predominating electrophilic influence.

There are, however, pyridone addition processes that are more readily rationalized and recognizable as 1,6-addition reactions. Thomas3 reported that 2-pyridone reacted with 2 equiv of butyllithium to give the 1,6-addition adduct in 55% yield; however, the use of other alkyllithium reagents was less successful. Joule4 found that when N-methyl-2-pyridone was treated with butyllithium, formation of a dimer was observed, which can be attributed to the 1,6-addition of a C-lithiated N-methyl2-pyridone to another mole of N-methyl-2-pyridone. Sosnicki5 has described the addition of alkyllithium-modified Grignard reagents to unsubstituted (or alkyl substituted) N-alkyl-2-pyridones which led predominantly to attack at C(6), but C(4) adducts were also observed. The analogous thiopyridones, though more reactive, displayed a similar regioselectivity trend.

We recently reported the first examples of the intramolecular nucleophilic 1,6-addition to a 2-pyridone, which

JOC Article SCHEME 1. Lupin Alkaloid Synthesis via Intramolecular 1,6-Addition of Lactam Enolates

involved a lactam enolate, and this chemistry has been applied effectively to provide an efficient and convergent entry to representative lupin alkaloids,6 including (þ)-cytisine 3 (the unnatural enantiomer), (()-anagyrine 4a, and (()thermopsine 4b (Scheme 1). (-)-Cytisine 3 is a potent and selective partial agonist for the R4β2 neuronal nicotinic acetylcholine receptor,7 and while structural details of the receptor-ligand interaction have yet to be defined, cytisine nevertheless provided the inspiration for the design of varenicline (Chantix), Pfizer’s smoking cessation agent.8

Access to novel structural variants of cytisine (and indeed other lupin alkaloids) offers an important and timely vehicle with which to probe both the nicotinic pharmacophore9 and, of particular interest in terms of cytisine (vs other wellknown full agonists such as nicotine), those structural characteristics that contribute to the partial agonist profile associated with cytisine. This paper describes studies which have explored the scope of this intramolecular 1,6-enolate addition process and provided insights into the mechanistic detail of this process. This work was also pursued with a view to downstream (3) Thomas, E. W. J. Org. Chem. 1986, 51, 2184–2191. (4) Meghani, P.; Joule, J. J. Chem. Soc., Perkin Trans. 1 1988, 1, 1–8. (5) Sosnicki, J. G. Tetrahedron Lett. 2005, 46, 4295–4298. Sosnicki, J. G. Tetrahedron Lett. 2006, 47, 6809–6812. (6) Gray, D.; Gallagher, T. Angew. Chem., Int. Ed. 2006, 45, 2419–2423. Botuha, C.; Galley, C. M. S.; Gallagher, T. Org. Biomol. Chem. 2004, 2, 1825–1826. (7) Jensen, A. A.; Frølund, B.; Liljefors, T.; Krogsgaard-Larsen, P. J. Med. Chem. 2005, 48, 4705–4745. (8) (a) Coe, J. W.; Brooks, P. R.; Vetelino, M. G.; Wirtz, M. C.; Arnold, E. P.; Huang, J. H.; Sands, S. B.; Davis, T. I.; Lebel, L. A.; Fox, C. B.; Shrikhande, A.; Heym, J. H.; Schaeffer, E.; Rollema, H.; Lu, Y.; Mansbach, R. S.; Chambers, L. K.; Rovetti, C. C.; Schulz, D. W.; Tingley, F. D.; O’Neill, B. T. J. Med. Chem. 2005, 48, 3474–3477. (b) Coe, J. W.; Brooks, P. R.; Wirtz, M. C.; Bashore, C. G.; Bianco, K. E.; Vetelino, M. G.; Arnold, E. P.; Lebel, L. A.; Fox, C. B.; Tingley, F. D.; Schulz, D. W.; Davis, T. I.; Sands, S. B.; Mansbach, R. S.; Rollema, H.; O’Neill, B. T. Bioorg. Med. Chem. Lett. 2005, 15, 4889–4897. (9) Glennon, R. A.; Dukat, M. Bioorg. Med. Chem. Lett. 2004, 14, 1841– 1844.

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JOC Article SCHEME 2. Cyclization Mechanism and Oxidation-Based Side Reactions

application of this chemistry to generate core variants of cytisine 3. Results and Discussion We previously reported that under thermodynamic (THF, reflux) conditions, intramolecular 1,6-addition of the enolate derived from 1 afforded tricyclic adduct 2r exclusively and in high (94%) yield.6 When lactam deprotonation/cyclization was carried out at lower temperatures (-78 °C, 0 °C, or rt) a mixture of diastereomeric adducts 2r and 2β were observed (Scheme 2). Initially, the reproducibility of this sequence was an issue as varying amounts of adducts 2r and 2β, which depended on the reaction conditions/solvent used, and several other products were observed. Conducting enolate formation and cyclization at lower temperatures (-78 °C, 2 h) produced a ∼1:1 ratio of 2r/2β in excellent yield, but at higher temperatures (from rt to reflux) and longer reaction times, 2r was obtained as either the predominant (at rt) or the exclusive (at reflux) product. These observations suggested that the 1,6addition (enolate 6 f 7) is a reversible process, and when pure 2β was resubjected to these basic reaction conditions, isomerization occurred to afford 2r/2β in a 7:10 ratio after 3 h at -78 °C. In addition, a small amount (5%) of the ringopened lactam 1 was also obtained under these isomerization conditions which demonstrated (i) a thermodynamic and a kinetic preference for 2r and 2β, respectively, and (ii) that (most reasonably) this equilibration involves ring-opening and reclosure, i.e., 2r/β f 7 f ring-opened 6 (which is trapped on workup to give 1) f 7 f 2r/β. Minor reaction products, the yields of which depended on scale and other difficult-to-define variables, were identified as the tricyclic R-hydroxypyridone 5a and pyridone 5b which would be accounted for by the trapping of the initially formed adduct 7 by trace (but variable) levels of peroxides/ oxygen present in alumina dried THF.10 Fragmentation of adducts 8a/b could occur via two different pathways leading to 5a and 5b with the driving force being restoration of the aromatic pyridone moiety. Oxidation products such as 5a and 5b were, however, completely suppressed by use of freshly distilled (from Na/benzophenone) THF, affording 2r and 2β cleanly, reproducibly, and in excellent yields (85-95%). R-Hydroxypyridone 5a is an attractive and potentially useful cytisine analogue, and the exploitation of this chem(10) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers, F. J. Organometallics 1996, 15, 1518–1520.

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Gallagher et al. SCHEME 3.

Non-Lactam Cyclization Precursors

istry will be reported in due course. More importantly though, pyridone 5b was a key intermediate in our previous route6 to cytisine 3 and was obtained by MnO2 oxidation of 2r. Thus, the ability to generate 5b directly without any need for isolation and conversion of 2r was clearly advantageous, and once this pathway had been characterized, 5b was obtained in 60% conversion (by 1H NMR) when the tricyclic enolate 7 was trapped with bis(trimethylsilyl)peroxide (i.e., E=Me3SiOOSiMe3, Scheme 2) followed (effectively) by loss of HOSiMe3.11 Variation of the Nucleophilic Component We have already demonstrated the versatility of the intramolecular 1,6-addition outlined in Scheme 1 in terms of related bicyclic lactam substrates which led to the synthesis of (()-anagyrine 4a and (()-thermopsine 4b, but the presence of a lactam/amide appears to be critical for successful 1,6-addition to a pendant 2-pyridone. Using a simple model system, the ability of range of alternative enolate precursors to achieve 1,6-addition to a 2-pyridone has been evaluated (Scheme 3). Ester 12 and ketone 13 were prepared by N-alkylation of 2-pyridone with bromides 9 and 10, respectively, and the “carbocytisine” keto-based precursor 14 was synthesized from the known acetal 11.12 A thioamide variant 15 was prepared from lactam 1 using Lawesson’s reagent.13 Exposure of ester 12 to LiHMDS in THF resulted in formation of the corresponding intermolecular Claisen condensation adduct in 22% yield. Ketones 13 and 14 failed to cyclize, even when exposed to a range of different temperatures (-78 °C, 0 °C, refluxing THF), and thiolactam 15 also proved ineffective as a substrate for cyclization; in this latter case, a D2O quench confirmed that thioenolate formation did occur. Given the equilibration process observed in the conversion of 1 to 2r/2β, our conclusion is that cyclization of 1 (Scheme 2) occurs because the product, the extended lactam enolate 7, is more stable than the simple lactam enolate 6. Therefore, the enhanced stability of enolates derived from 12-15 (compared to an extended lactam enolate) favors the ring-opened form. (11) Adam, W.; Korb, M. N. Tetrahedron 1996, 52, 5487–5494. (12) De Loos, W. A. J.; van den Berg-Van Kuijk, A. J. W.; van Iersel, H.; de Haan, J.; Buck, H. Recl. Trav. Chim. Pay. B. 1980, 99, 53–57. (13) Full details of the synthesis of these compounds is described in the Supporting Information.

JOC Article

Gallagher et al. SCHEME 4. Cyclization Precursors for 5-, 6-, and 7-Membered Ring Formation

Variation of Ring Size Since the intramolecular 1,6-addition had only been used to make six-membered rings, the question remained as to whether other ring sizes (which provide the opportunity to access, for example, nor- or homobispidines) were accessible by this method. A series of amide-based precursors 16a-c were synthesized by N-alkylation of 2-pyridone with the commercially available ω-bromoester and subsequent exposure to pyrrolidine (Scheme 4).13 As expected, the six-membered cyclization product 17b formed readily and in good yield. Differing from the cytisine analogues 2r/2β, 17b formed as a single diastereomer which was also resistant to subsequent oxidation (MnO2) to form the corresponding pyridone. Five-ring formation was also facile, and 16a cyclized readily upon treatment with LiHMDS to provide adduct 17a. However, and as might be anticipated, formation of a seven-membered ring via this method, though achieved, was inefficient, and only small

amounts of 17c were observed, despite screening a range of bases and temperatures.14 Given the success of the formation of the 5-membered adduct 17a, this method was extended to the synthesis of norbispidine 23 (Scheme 5). Following a procedure described by Stanetty,15 reaction of itaconic acid 18 with benzylamine yielded lactam 19 in excellent yield. Subsequent esterification, reduction, and bromination yielded bromide 20, which underwent N-alkylation with 2-pyridone, providing the cyclization precursor 21. Conversion of 21 to diastereomers 22r and 22β (as a 1:3 ratio) occurred in essentially quantitative yield, and MnO2 oxidation of this mixture yielded norbispidine 23 (seven steps, 19% overall yield). Analogous to the bispidine system (Scheme 2), a direct quench of the enolate resulting from cyclization of 21 with bis(trimethylsilyl)peroxide led to the one-pot formation of pyridone 23 in 60% conversion (by 1H NMR). Variation of the Heterocyclic Acceptor While lactam enolates added readily to pyridones to form five- or six-membered rings, 1,6-addition to other “acceptor” electrophiles, such as those shown in Scheme 6, have proven more challenging. The lactam-precursors 25a/b and 26a/b were synthesized directly by N-alkylation of the appropriate heterocycles with bromide 246 and substrates 27a/b were synthesized by N-alkylation with 9 and subsequent aminolysis with pyrrolidine. The requisite heterocyclic precursors were commercially available16 except for pyridopyrazinone

SCHEME 5.

Application of a Five-Membered Cyclization to Nor-bispidine 23

SCHEME 6.

Synthesis of Cyclization Precursors with Different “Acceptor” Heterocycles

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JOC Article SCHEME 7.

a

Gallagher et al.

In Situ IR Profile of the Cyclization of 1a

Key: (A) solvent background (individual spectrum not shown); (B) lactam 1 in THF; (C) addition of LiHMDS; (D) quench NH4Cl(s) then MeOH.

30, which was prepared from the literature known pyridopyrazine 28.17 N-Oxidation of 28 gave 2918 which, following acetylation and rearrangement, gave the target pyridopyrazinone 30 in good yield.13 Isoquinolone analogues 25a and 27a were unreactive toward cyclization at lower temperatures and decomposed under more forcing reflux conditions. Clearly, some disruption of aromaticity within the isoquinolone moiety would be associated with nucleophilic addition to these systems, which may have the effect of favoring the ring-open enolate (cf. 6 vs 7, Scheme 2). This, again points to the 1,6addition as a relatively finely balanced process that may require an additional factor(s) to stabilize/favor the cyclized product.19 More electron-deficient pyridone variants might be expected to promote cyclization; however, pyridopyrazinones (14) LiHMDS/THF (-78 °C, 0 °C, rt, reflux), n-BuLi/THF (-78 °C), KO-t-Bu/DMF (110 °C). (15) Stanetty, P.; Turner, M.; Mihovilovic, M. D. Molecules 2005, 10, 367–375. (16) 2-Pyrazinone, though commercially available, has been synthesized from serine ethyl ester by aminolysis with ammonia and subsequent condensation with glyoxal. (17) Pyridopyrazine 28 was synthesized in two steps by (i) catalytic hydrogenation of 4-amino-3-nitropyridine and (ii) subsequent condensation with glyoxal as described by: (a) Clark-Lewis, J. W.; Singh, R. J. Chem. Soc. 1962, 2379–2382. (b) Clark-Lewis, J. W.; Singh, R. J. Chem. Soc. 1962, 3162–3167. (18) Oxidation of pyridopyrazine 28 with p-nitroperbenzoic acid has been described in: Boutte, D.; Queguiner, G.; Pastour, P. C. R. Seances Acad. Sci. C 1971, 273, 1529–1532. (19) A plausible alternative explanation for the apparent “lack of reaction” associated with 25a is that this substrate undergoes preferentially 1,2addition (to the isoquinolone CdO); see intermediate 33, Scheme 8. This would not result in a disruption of the benzenoid aromaticity that would accompany 1,6-addition. Two experiments were carried out to probe possible fates of 25a. Exposure of 25a to LiHMDS followed by a D2O quench showed no incorporation of D in recovered 25a. Second, in the event that lactam enolate formation and cyclization were facile processes, we used (TMSO)2 to trap the putative extended enolate (and cyclized) product (compare 7f8f5a/b, Scheme 2). No evidence for an oxidized product analogous to 5a (or a regioisomer) or 5b was obtained, and only 25a was recovered. It is difficult to understand why enolization should be particularly inhibited in the case of lactam 25a, and these experiments cannot exclude the possibility that cyclization of 25a does, nevertheless, occur (1,2- or 1,6-addition) but that the product is unstable with respect to ring opening on work up.

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25b and 27b proved to be highly reactive and underwent rapid decomposition when exposed to basic conditions, even at low temperatures. To establish if this reactivity was associated with formation and reaction of a lactam enolate, the n-butyl analogue 31 was exposed to LiHMDS at -78 °C. This substrate also underwent rapid decomposition suggesting that these highly electron-deficient heterocycles are themselves excessively susceptible to nucleophilic attack. More interestingly, because of the potential of this chemistry to provide access to fundamentally different core variants of cytisine, pyrimidinone 26a and pyrazinone 26b also proved problematic. In neither case was any reaction (apparently) observed at -78 °C, and at higher temperatures degradation of both 26a and 26b occurred according to 1H NMR analysis of the crude reaction product. This observation was, however, at odds with our rationale of the 1,6-addition equilibrium, since the more electrondeficient heterocycle was anticipated to stabilize the addition adduct (cf. the earlier work of Tohda1 and Hiroya2). We suspected a competing and reversible reaction, for example, formation of an unstable (with respect to the protic quench) 1,2-addition, and in situ IR spectroscopy provided a flexible means by which to probe the nature of the enolate/cyclized species generated under these basic conditions. To provide a baseline for the study of these problematic cases, initial spectroscopic studies focused on the conversion of 1 (via enolates 6 and 7) to 2r/β since these latter products had already been fully characterized. After addition of LiHMDS to a solution of 1 in THF, distinctive shifts for the carbonyl moieties were observed and assessed (a) as indicators of formation of the lactam enolate 6, (b) as indicators that a reaction was occurring (via perturbation of the heteroarene carbonyl functionality), and (c) to support generation of the product lactam enolate 7. In situ IR profiles of the cyclization of 1 were collected to compare the behavior of this well-understood substrate with that of pyrimidinone 26a under the same reaction conditions. Due to instrument limitations, reactions involving both 1 and 26a were carried out at -20 °C at which temperature formation of reaction

JOC Article

Gallagher et al. SCHEME 8.

a

In Situ IR Profile of the Attempted Cyclization of 26aa

Key: (A/B) solvent with undissolved 26a; (C) addition of LiHMDS; (D) quench NH4Cl(s) then MeOH.

intermediates occurred immediately. Schemes 7 and 8 outline the chemistry associated with 1 and 26a, respectively, as well as the snapshot in situ IR spectra of the intermediates (as assigned) that formed during each reaction.20 Background spectra of the solvent provided a window (1500-1800 cm-1) to monitor changes in the carbonyl functionalities during reaction. Lactam 1 displayed two carbonyl signals (1647 and 1682 cm-1) which were identified as the piperidinone lactam [C(13)dO] and pyridone [C(2)dO]21 carbonyls, respectively (Scheme 7, Spectrum B). After the system containing 1 had reached steady state, 2 equiv of LiHMDS was added over 1 min, and the initially colorless solution instantly turned bright yellow. Both carbonyl signals associated with 1 diminished to be replaced by new peaks that were assigned to 7: 1645 cm-1 [C(13)dO], (20) Instrumentation and Software. The in situ IR reaction spectra were collected using a MCT (mercury cadmium telluride) detector; diamond composite probe connected via AgX 9.5 mm x 2 m fiber (silver halide); sampling 2000 to 650 at 8 cm-1 resolution. The MCT detector operates at liquid nitrogen temperature. (21) These IR signals were consistent with the neat IR spectrum of lactam 1. (22) The frequencies for simple nonconjugated amide enolates have been reported as 1575, 1540, 1604 cm-1. See: McWilliams, C.; Reamer, R. Process Chemistry in the Pharmaceutical Industry, 1st ed.; Gadamasetti, K., Braish, T., Eds.; CRC Press: Boca Raton, 2007; Vol. 2, Chapter 20, p 320.

1522 cm-1 [LiO-C(2)dC(3)],22 and 1244 cm-1 (HMDS)23 (Spectrum C). After 25 min, a slow quench was initiated with solid ammonium chloride, which was then accelerated by addition of methanol. At this point, IR signals associated with 7 were replaced by a broad peak at 1638 cm-1, attributed to the two similar lactam carbonyls in 2 [C(13)dO] and [C(2)dO] (Spectrum D).24 NMR analysis of the crude reaction mixture subsequently confirmed clean formation of a 3:1 mixture of 2r and 1. (The relatively larger amount of 1 isolated likely reflects the nature of the quench conditions used as compared to the more usual synthetic procedure). The same reaction conditions were then applied to the pyrimidinone substrate 26a (Scheme 8). No IR signals were observed for pyrimidinone 26a prior to addition of base as this substrate was only sparingly soluble in THF at -20 °C. However, following addition of 2 equiv of LiHMDS, dissolution of 26a occurred to give a bright yellow solution (as was observed with 1), but and surprisingly, no sharp/strong IR signals were observed in the region of 1500-1800 cm-1. The absence of CdO signals leads us to deduce that (a) an enolate derived from lactam 26a had formed and (b) a (23) An IR spectrum of HMDS is available at http://www.Acros.com. (24) These IR signals were consistent with the neat IR spectrum of lactam 2r.

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JOC Article SCHEME 9.

Isolation of the HMDS Adduct 35

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1,6-addition process now more clearly defined, open the way to tuning the process (via subtle changes to the precise structure of the heteroaromatic component) to broaden the applicability of this chemistry. Experimental Section

reaction had occurred that involved the pyrimidinone CdO but (c) no new bispidine-like lactam, such as the expected intermediate 32 or the 1,2-addition-product 33, was generated. After 40 min, the reaction was quenched (solid ammonium chloride followed by methanol) which led to rapid generation of a series of carbonyl signals, among which were those associated with 26a (1663 cm-1 [C(13)dO] and 1635 cm-1 [C(2)dO]). NMR analysis of the crude reaction mixture confirmed the presence of 26a as the major component as well as several unidentified but minor degradation products. From these observations, we conclude that although deprotonation of the piperidinone lactam 26a occurred, a competing process, involving nucleophilic addition of HMDS to the electron-deficient pyrimidinone, was preferred leading to adducts (some or all) of type 34. Although most of the signals associated with these possible intermediates appear to be masked by the solvent background, an increased intensity of the broad solvent signal around 1461 cm-1 suggests the presence of more signals in this region. Based on recovery of 26a, we suggest that adducts 34a-c are unstable with respect to loss of HMDS upon protic quench. Credible support for a process involving addition of HMDS to the pyrimidinone moiety was provided by the isolation of adduct 35 after reaction of 26a with NaHMDS at -78 °C (Scheme 9). Adduct 35 was obtained as a 1:1 mixture of diastereomers in 30% yield and was characterized by 1 H/13C NMR; this adduct proved to be relatively unstable with respect to reversion in solution to 26a. Conclusion In conclusion, the intramolecular 1,6-addition of a lactam enolate to a pyridone, which is highly effective as the basis of a strategy for the synthesis of the lupin alkaloids, is a finely balanced process. Equilibration of adducts 2r/β occurs via a reversible ring-opening/1,6-addition, and although this reaction will tolerate variation of the lactam unit,25 other more stabilized enolates (derived from an ester, ketone, or thiolactam) shift this equilibrium toward the ring-opened form. Further, it has been shown that this chemistry is applicable in the synthesis of five-membered as well as sixmembered rings, and this has been extended to the synthesis of a nor-bispidine core (23). Changes associated with the electrophilic heteroarene “acceptor” component have a major impact on the reaction outcome. Although attempts to promote cyclization by use of a more electron-deficient acceptor failed, alternative preferred pathways have been identified and validated by IR studies. The insights garnered from these studies, and with the scope and limitations of this (25) In addition to the substrates shown, a series of 4-substituted pyridones variants of 1, leading to the corresponding 4-substituted cytisine analogues, have also been successfully employed in this chemistry.

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General experimental procedures are described in the Supporting Information. (()-(2R,1S,9S)-11-Benzyl-7,11-diazatricyclo[7.3.1.02,7]tridec-3ene-6,12-dione (2β) and (()-(2S,1S,9S)-11-Benzyl-7,11-diazatricyclo[7.3.1.02,7]tridec-3-ene-6,12-dione (2r). To a solution of cyclization precursor 1 (130 mg, 0.44 mmol)6 in THF (5 mL, freshly distilled from sodium and benzoquinone) at -78 °C was added LiHMDS (1.0 M THF solution, 0.84 mL, 0.84 mmol) dropwise. The reaction solution was stirred at -78 °C for 2 h and then quenched by the addition of saturated aq NH4Cl (5 mL). The aqueous phase was extracted with EtOAc (5 mL) and CH2Cl2 (5 mL), and then the combined organic washings were dried (Na2SO4) and concentrated in vacuo. Purification by flash chromatography [CH2Cl2/MeOH (99:1)] yielded 2β (15 mg, 11%) as a colorless oil: Rf 0.45 [CH2Cl2/MeOH (10:1)]; 1H NMR (400 MHz, CDCl3) δH 7.21-7.46 (5H, m), 5.84 (1H, dtd, J=10.0, 3.0, 2.0 Hz), 5.79 (1H, dtd, J=10.0, 2.0, 1.5 Hz), 4.99 (1H, dd, J=14.0, 10.5 Hz), 4.93 (1H, d, J=14.0 Hz), 4.32 (1H, d, J=14.0 Hz), 4.28 (1H, ddt, J=7.5, 5.5, 2.0 Hz), 3.36 (1H, dd, J= 12.0, 3.5 Hz), 2.99-3.04 (2 H, m), 2.97 (1H, dt, J=12.0, 2.0 Hz), 2.72 (1H, td, J=3.5, 2.0 Hz), 2.43-2.52 (1H, m), 2.10 (1H, dd, J = 14.0, 6.0 Hz), 1.58-1.76 (2H, m); 13C NMR (101 MHz, CDCl3) δC 171.8, 167.2, 136.7, 128.7, 128.4, 127.7, 126.4, 122.8, 59.5, 51.8, 49.9, 43.3, 40.8, 31.6, 25.3, 21.2; IR νmax/cm-1 (film) 2934 (s), 2239 (w), 1635 (s), 1454 (m), 1262 (s), 1075 (m), 726 (s); MS m/z ESIþ 297 ([M þ H]þ, 100), 319 ([M þ Na]þ, 100); HRMS C18H21N2O2 requires 297.1598, found [M þ H]þ = 297.1597. NOE interactions outlined in the Supporting Information confirmed the relative stereochemistry of 2β. Continued elution [CH2Cl2/MeOH (99:1)] yielded 2r (30 mg, 22%) as a colorless oil: Rf 0.35 [CH2Cl2/MeOH (10:1)]; 1H NMR (400 MHz, CDCl3) δH 7.19-7.33 (5H, m), 5.75-5.84 (2H, m), 4.89 (1H, dt, J=13.5, 2.5 Hz), 4.55 (1H, d, J=14.5 Hz), 4.48 (1H, d, J=14.5 Hz), 4.13-4.17 (1H, m), 3.35 (1H, dd, J= 13.0, 6.5 Hz), 3.24 (1H, d, J = 13.0), 2.84-2.93 (1H, m), 2.77-2.81 (2H, m), 2.72-2.75 (1H, m), 2.23-2.28 (1H, m), 2.18 (1H, ddd, J = 13.0, 5.5, 3.5 Hz), 2.00-2.05 (1H, m); 13C NMR (101 MHz, CDCl3) δC 167.9, 166.9, 136.7, 128.6, 128.2, 127.5, 123.6, 121.9, 60.2, 51.2, 50.0, 47.2, 43.9, 31.3, 28.9, 27.6. Spectroscopic data for this compound were consistent with those reported previously.6 Intermediary fractions collected afforded a 1:1 mixture of diastereomers (∼50% yield by mass). Complete separation of these isomers required multiple chromatography. (()-11-Benzyl-7,11-diazatricyclo[7.3.1.02,7]tridec-2,4-diene6,12-dione (5b) via Tandem Cyclization-Oxidation. To a solution of cyclization precursor 1 (170 mg, 0.57 mmol)6 in THF (10 mL, freshly distilled from sodium and benzoquinone) at -78 °C was added LiHMDS (1.0 M THF solution, 1.00 mL, 1.00 mmol) dropwise. The reaction solution was heated at reflux for 2 h and then cooled to rt. Freshly distilled bis(trimethylsilyl)peroxide (200 mg, 1.12 mmol, bp 42-44 °C/30 mmHg, lit.11 bp 40 °C/30 mmHg) was then added via a glass pipet, and the reaction solution was stirred for 18 h at rt. After this time, the reaction was quenched by the addition of saturated aq NH4Cl (10 mL) and extracted with EtOAc (3  10 mL) and CH2Cl2 (10 mL). The combined organic washings were dried (Na2SO4) and concentrated in vacuo. The crude reaction mixture was analyzed by 1H NMR to show ∼60% conversion to 5b: Rf 0.15 [CH2Cl2/ MeOH (19:1)]; 1H NMR (400 MHz, CDCl3) δH 7.33 (1H, dd, J= 9.0, 7.0 Hz), 7.25-7.32 (3H, m), 7.10 (2H, dd, J=7.5, 2.0 Hz),

Gallagher et al. 6.52 (1H, dd, J=9.0, 1.5 Hz), 6.36 (1H, dd, J=7.0, 1.5 Hz), 4.64 (1H, d, J=14.5 Hz), 4.41 (1H, d, J=14.5 Hz), 4.07 (1H, dt, J= 15.5, 1.0 Hz), 3.97 (1H, ddd, J=15.5, 6.5, 1.0 Hz), 3.77 (1H, dd, J=4.5, 3.0 Hz), 3.56 (1H, ddd, J=13.0, 6.0, 1.0 Hz), 3.20 (1H, dt, J = 13.0, 1.5 Hz), 2.75-2.83 (1H, m), 2.20-2.28 (1H, m), 2.06-2.16 (1H, m); 13C NMR (101 MHz, CDCl3) δC 167.3, 163.4, 144.0, 139.2, 136.1, 128.8, 127.9, 127.7, 118.3, 106.5, 52.9, 50.0, 49.7, 43.2, 25.7, 23.1. Spectroscopic data for this compound were consistent with those reported previously.6 (()-(1S,8aR)-1-(Pyrrolidine-1-carbonyl)-1,2,3,8a-tetrahydroindolizin-5(6H)-one (17a). To a solution of cyclization precursor 16a (300 mg, 1.27 mmol) in THF (15 mL, freshly distilled from sodium and benzoquinone) at -78 °C was added LiHMDS (1.0 M THF solution, 2.55 mL, 2.55 mmol). The reaction solution was stirred at -78 °C for 4 h and then quenched by the addition of saturated aq NH4Cl (50 mL). The aqueous phase was extracted with CH2Cl2 (4  50 mL), and then the combined organic washings were dried (Na2SO4) and concentrated in vacuo. Purification by flash chromatography [CH2Cl2/MeOH (49:1-24:1)] yielded 17a (219 mg, 73%) as a colorless oil: Rf 0.27 [CH2Cl2/MeOH (9:1)]; 1H NMR (500 MHz, CDCl3) δH 5.74-5.85 (2H, m), 4.41-4.50 (1H, m), 3.87 (1H, dt, J=12.0, 9.0 Hz), 3.39-3.56 (5H, m), 2.94-3.03 (1H, m), 2.89 (1H, dt, 21.5, 4.0 Hz), 2.69 (1H, dt, J=11.0, 7.5 Hz), 2.16-2.26 (1H, m), 2.02-2.12 (1H, m), 1.98 (2H, quin, J=6.5 Hz), 1.89 (2H, quin, J=7.0 Hz); 13C NMR (125 MHz, CDCl3) δC 169.3, 166.0, 123.4, 123.0, 61.4, 48.2, 46.5, 45.9, 43.2, 32.5, 26.3, 25.9, 24.1; IR νmax/ cm-1 (film) 2972 (w), 2881 (w), 1632 (s), 1447 (m), 1411 (w), 1346 (w), 691 (w); MS m/z ESIþ 257 ([M þ Na]þ, 100); HRMS C13H18N2O2Na requires 257.1260, found [M þ Na]þ =257.1266. NOE interactions outlined in the Supporting Information confirmed the relative stereochemistry of 17a. (()-(1S,9aR)-1-(Pyrrolidine-1-carbonyl)-3,4,7,9a-tetrahydro1H-quinolizin-6(2H)-one (17b). To a solution of cyclization precursor 16b (200 mg, 0.81 mmol) in THF (5.0 mL) at -78 °C was added LiHMDS (1.0 M THF solution, 1.61 mL, 1.61 mmol). The reaction solution was stirred at -78 °C for 1 h then quenched via the addition of saturated aq NH4Cl solution (25 mL). The aqueous phase was extracted with EtOAc (4  25 mL) and CH2Cl2 (4  25 mL), and then the combined organic washings were dried (Na2SO4), filtered, and concentrated in vacuo. Purification by column chromatography [CH2Cl2/MeOH (49:1)] yielded 17b (165 mg, 83%) as an off-white solid: Rf 0.27 [CH2Cl2/MeOH (19:1)]; mp 121 °C (benzene/n-hexane); 1H NMR (500 MHz, CDCl3) δH 5.62-5.78 (2H, m), 4.90 (1H, ddt, J=13.0, 4.0, 2.0 Hz), 4.16-4.26 (1H, m), 3.44-3.54 (3H, m), 3.39 (1H, dt, J=10.0, 6.5 Hz), 2.88-3.04 (2H, m), 2.56 (1H, td, J= 13.0, 3.0 Hz), 2.39 (1H, ddd, J=12.0, 10.5, 4.0 Hz), 1.77 - 2.03 (7H, m), 1.50 (1H, qt, J=13.0, 4.0 Hz); 13C NMR (101 MHz, CDCl3) δC 171.2, 166.0, 123.6, 122.4, 59.6, 49.1, 46.7, 45.7, 41.9, 31.7, 28.3, 26.0, 24.31, 24.30; IR νmax/cm-1 (film) 2951 (m), 2885 (m), 1635 (s, br), 1478 (s), 1440 (s), 1407 (m), 1341 (m), 1322 (m), 1265 (m), 1092 (m); MS m/z EIþ 150 ([C9H12NO]þ, 100), 248 ([M]þ, 50); HRMS C14H20N2O2 requires 248.1525, found [M]þ= 248.1537. NOE interactions outlined in the Supporting Information confirmed the relative stereochemistry of 17b. (()-(10S,10aR)-10-(Pyrrolidine-1-carbonyl)-6,7,8,9,10,10ahexahydropyrido[1,2-a]azepin-4(3H)-one (17c). To a solution of cyclization precursor 16c (112 mg, 0.42 mmol) in THF (5.0 mL) at -78 °C was added LiHMDS (1.0 M solution in THF, 850 μL, 0.85 mmol). The reaction solution was stirred at -78 °C for 5 h and then quenched via the addition of saturated aq NH4Cl solution (40 mL). The aqueous phase was extracted with CH2Cl2 (4  40 mL), and then the combined organic washings were dried (Na2SO4), filtered, and concentrated in vacuo. Purification by column chromatography [CH2Cl2/MeOH (9:1)] followed by secondary purification by column chromatography [EtOAc/MeOH (19:1)] yielded 17c (15 mg, 14%) as an off-white

JOC Article solid: Rf 0.23 [CH2Cl2/MeOH (19:1)]; mp 148-152 °C (benzene/ n-hexane); 1H NMR (500 MHz, CDCl3) δH 5.83-5.89 (1H, m), 5.77 (1H, ddd, J=10.0, 4.0, 3.0 Hz), 4.45 (1H, ddd, J=14.0, 7.0, 2.5 Hz), 4.32 (1H, ddt, J = 9.5, 5.0, 2.5 Hz), 3.44 (4H, m), 2.96-3.02 (2H, m), 2.92 (1H, m, J=14.0, 11.0, 6.0 Hz), 2.43 (1H, td, J = 9.5, 4.5 Hz), 2.12 (1H, dtt, J = 13.5, 11.0, 7.0 Hz), 1.74-2.01 (7H, m), 1.57-1.67 (1H, m), 1.38-1.49 (1H, m); 13C NMR (126 MHz, CDCl3) δC 171.7, 168.0, 126.8, 122.9, 62.8, 53.6, 46.8, 46.6, 45.7, 32.4, 29.3, 26.0, 25.5, 24.3, 24.0; IR νmax/ cm-1 (film) 2935 (w), 1635 (s), 1592 (m), 1539 (w), 1471 (m), 1432 (m), 1401 (w), 1345 (w), 1320 (w), 1257 (w), 1065 (w), 808 (w), 706 (w); MS m/z EIþ 126 (100%), 262 ([M]þ, 60); HRMS C15H22N2O2 requires 262.1681, found [M]þ = 262.1680. NOE interactions outlined in the Supporting Information confirmed the relative stereochemistry of 17c. (()-1-Benzyl-4-bromomethylpyrrolidin-2-one (20). To a solution of 1-benzyl-4-(hydroxymethyl)pyrrolidin-2-one (6.11 g, 29.77 mmol)15 in toluene (100 mL) at 0 °C was added PBr3 (2.93 mL, 31.26 mmol) dropwise. The reaction mixture was heated at reflux for 5 h, cooled to 0 °C, and quenched by dropwise addition of H2O (5 mL). Concentration in vacuo afforded an orange gum which was dissolved in EtOAc (50 mL) and sonicated to aid dissolution. The solution was then partitioned between H2O (30 mL) and EtOAc (3  20 mL). The combined organic extracts were dried (Na2SO4) and concentrated in vacuo to afford bromide 20 (5.11 g, 64%) as a colorless oil: Rf 0.35 (EtOAc); 1H NMR (400 MHz, CDCl3) δH 7.22-7.45 (5H, m), 4.52 (1H, d, J=14.5 Hz), 4.44 (1H, d, J=14.5 Hz), 3.46 (1H, dd, J=10.0, 0.5 Hz), 3.44 (1H, dd, J=10.0, 3.0 Hz), 3.37 (1H, dd, J = 10.0, 7.5 Hz), 3.11 (1H, dd, J = 10.0, 6.0 Hz), 2.73-2.86 (1H, m), 2.68 (1H, ddd, J =17.0, 9.0, 0.5 Hz), 2.35 (1H, dd, J = 17.0, 7.0 Hz); 13C NMR (101 MHz, CDCl3) δC 172.9, 136.1, 128.8, 128.2, 127.7, 51.1, 46.6, 36.6, 35.7, 33.5; IR νmax/cm-1 (film) 2922 (m), 1685 (s), 1496 (m), 1445 (m), 1420 (m), 1358 (w), 1296 (m); MS m/z ESIþ 268 ([M{79Br} þ H]þ, 77), 270 ([M{81Br} þ H]þ, 80), 290 ([M{79Br} þ Na]þ, 100), 292 ([M{81Br}þNa]þ, 90); HRMS C12H15NO79Br requires 268.0331, found [M þ H]þ =268.0343. (()-1-((1-Benzyl-5-oxopyrrolidin-3-yl)methyl)pyridin-2(1H)one (21). A stirred mixture of 2-pyridone (2.61 g, 27.40 mmol), bromide 20 (7.99 g, 30.14 mmol), K2CO3 (8.33 g, 60.30 mmol), and tetrabutylammonium bromide (0.89 g, 2.75 mmol) in water (1 mL) and toluene (100 mL) was heated at reflux overnight. After being cooled to rt, the reaction mixture was filtered and concentrated in vacuo. Flash chromatography [CH2Cl/MeOH (19:1)] yielded 21 (5.15 g, 61%) as a colorless oil: Rf 0.40 [CH2Cl2/MeOH (19:1)]; 1H NMR (400 MHz, CDCl3) δH 7.20-7.41 (6H, m), 6.82 (1H, dd, J = 7.0, 1.5 Hz), 6.54 (1H, dd, J=9.0, 0.5 Hz), 6.05 (1H, td, J=7.0, 1.5 Hz), 4.54 (1H, d, J= 14.5 Hz), 4.36 (1H, d, J=14.5 Hz), 3.87 (1H, dd, J=13.0, 7.0 Hz), 3.79 (1H, dd, J=13.0, 8.0 Hz), 3.33 (1H, dd, J=10.0, 7.0 Hz), 2.93-3.06 (2H, m), 2.62 (1H, dd, J=17.0, 8.0 Hz), 2.24 (1H, dd, J=17.0, 5.5 Hz); 13C NMR (101 MHz, CDCl3) δC 179.0, 172.9, 139.7, 137.6, 136.4, 128.8, 128.4, 127.8, 121.3, 106.0, 53.1, 48.9, 46.5, 35.3, 30.8; IR νmax/cm-1 (film) 2918 (s), 2851 (s), 1681 (s), 1657 (s), 1586 (s), 1541 (m), 1429 (s), 1255 (m), 1155 (m); MS m/z ESIþ 283 ([M þ H]þ, 10), 305 ([M þ Na]þ, 100); HRMS C17H18N2O2Na requires 305.1267, found [MþNa]þ=305.1260. (()-(3aS,9aR,9bS)-2-Benzyl-2,3,3a,7,9a,9b-hexahydro-1Hpyrrolo[3,4-a]indolizine-1,6(4H)-dione (22β) and (()-(3aS,9aS, 9bS)-2-Benzyl-2,3,3a,7,9a,9b-hexahydro-1H-pyrrolo[3,4-a]indolizine-1,6(4H)-dione (22r). To a solution of cyclization precursor 21 (786 mg, 2.78 mmol) in THF (30 mL, freshly distilled from sodium and benzoquinone) at -78 °C was added LiHMDS (1.0 M THF solution, 5.6 mL, 5.60 mmol) dropwise. The reaction solution was stirred at -78 °C for 2 h and then quenched by the addition of saturated aq NH4Cl (10 mL). The aqueous phase was extracted with EtOAc (2  15 mL) and CH2Cl2 (15 mL), and J. Org. Chem. Vol. 75, No. 11, 2010

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JOC Article then the combined organic washings were dried (Na2SO4) and concentrated in vacuo. Purification by flash chromatography [CH2Cl2/MeOH (19:1)] yielded adduct 22β (200 mg, 28%) as a colorless oil: Rf 0.25 [CH2Cl2/MeOH (19:1)] 1H NMR: (500 MHz, CDCl3) δH 7.28-7.38 (3H, m), 7.22-7.26 (2H, m), 6.09-6.13 (1H, m), 5.84-5.89 (1H, m), 4.58 (1H, dd, J=12.0, 8.5 Hz), 4.54 (1H, d, J = 14.5 Hz), 4.41 (1H, d, J = 14.5 Hz), 4.24-4.31 (1H, m), 3.49 (1H, dd, J=10.5, 8.0 Hz), 3.09 (1H, dd, J=10.5, 3.5 Hz), 3.03 (1H, dd, J=10.0, 7.5 Hz), 2.84-3.00 (3H, m), 2.79 (1H, dd, J=12.0, 7.5 Hz); 13C NMR (125 MHz, CDCl3) δC 172.9, 165.7, 135.7, 128.7, 128.0, 127.8, 124.8, 123.0, 60.1, 52.9, 49.6, 49.2, 46.6, 32.8, 32.1; IR νmax/cm-1 (film) 2922 (m), 2851 (m), 1684 (s), 1646 (s), 1496 (m), 1446 (s), 1314 (m), 1261 (m), 729 (w); MS m/z ESIþ 283.1 ([M þ H]þ, 33), 305.1 ([M þ Na]þ, 100); HRMS C17H18N2O2Na requires 305.1260, found [M þ Na]þ = 305.1267, C17H19N2O2 requires 283.1441, found [MþH]þ =283.1449. Continued elution [CH2Cl2/MeOH (9:1)] yielded diastereomer 22r (100 mg, 14%) as a colorless oil: Rf 0.50 [CH2Cl2/MeOH (9:1)]; 1H NMR (400 MHz, CDCl3) δH 7.07-7.38 (5H, m), 6.30-6.36 (1H, m), 5.81-5.90 (1H, m), 4.44 (1H, d, J=15.0 Hz), 4.35 (1H, d, J=15.0 Hz), 4.08-4.17 (1H, m), 3.48 (1H, dd, J= 11.0, 9.0 Hz), 3.34 (1H, dd, J=12.5, 7.5 Hz), 3.27 (1H, t, J= 8.0 Hz), 2.92-3.04 (4H, m), 2.66 (1H, q, J=7.5 Hz); 13C NMR (125 MHz, CDCl3) δC 171.4, 166.0, 135.7, 128.7, 128.3, 127.9, 127.8, 122.2, 60.0, 52.2, 50.1, 50.0, 46.6, 32.4, 32.1; IR νmax/cm-1 (film) 3152 (m), 3055 (m), 2925 (m), 1684 (s), 1647 (s), 1585 (m), 1496 (m), 1447 (s), 1322 (m), 1279 (m), 1077 (w); MS m/z ESIþ 283 ([M þ H]þ, 100); HRMS C17H19N2O2 requires 283.1441, found [M þ H]þ = 283.1449. Note: Intermediary fractions collected afforded a 3:1 mixture of diastereomers 22β and 22r (∼60% yield by mass). (()-2-Benzyl-2,3,3a,9b-tetrahydro-1H-pyrrolo[3,4-a]indolizine1,6(4H)-dione (23) via MnO2 Oxidation. To adducts 22β and 22r (930 mg, 3.29 mmol, as a 3:1 mixture of 22β/r) in DCE (50 mL) was added MnO2 (2.86 g, 32.9 mmol). The reaction mixture was heated at 90 °C for 18 h and then cooled to rt, filtered through a plug of Celite, and concentrated in vacuo to afford pyridone 23 (627 mg, 68%) as a pale brown microcrystalline solid: Rf 0.35 [CH2Cl2/MeOH (19:1)]; mp 203-206 °C (toluene); 1H NMR (500 MHz, CDCl3) δH 7.41 (1H, dd, J=9.0, 7.0 Hz), 7.27-7.35 (3H, m), 7.17-7.20 (2H, m), 6.50 (1H, dt, J=7.0, 1.0 Hz), 6.48 (1H, dt, J=9.0, 1.0 Hz), 4.48 (1H, d, J=14.5 Hz), 4.43 (1H, d, J= 14.5 Hz), 4.42 (1H, dd, J=13.5, 8.5 Hz), 4.14 (1H, dt, J=9.0, 1.0 Hz), 3.92 (1H, dd, J=13.5, 5.0 Hz), 3.62 (1H, dd, J=10.5, 8.0 Hz), 3.28-3.36 (1H, m), 3.15 (1H, dd, J=10.5, 4.0 Hz); 13C NMR (125 MHz, CDCl3) δC 169.2, 162.1, 147.9, 135.3, 128.8, 127.9, 127.7, 118.8, 102.6, 54.3, 52.4, 50.8, 47.3, 31.2; IR νmax/ cm-1 (film) 2924 (m), 1695 (s), 1662 (s), 1582 (s), 1543 (w), 1496 (w), 1447 (s), 1262 (m), 1154 (m); MS m/z ESIþ 281 ([MþH]þ, 100); HRMS C17H17N2O2 requires 281.1284, found [MþH]þ= 281.1290. (()-2-Benzyl-2,3,3a,9b-tetrahydro-1H-pyrrolo[3,4-a]indolizine1,6(4H)-dione (23) via Tandem Cyclization Oxidation. To a solution of cyclization precursor 21 (116 mg, 0.41 mmol) in

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Gallagher et al. THF (10 mL, freshly distilled from sodium and benzoquinone) at -78 °C was added LiHMDS (1.0 M THF solution, 0.82 mL, 0.82 mmol) dropwise. The reaction solution was stirred at -78 °C for 2 h, and then freshly distilled bis(trimethylsilyl)peroxide (146 mg, 0.82 mmol, bp 42-44 °C/30 mmHg, lit.11 bp 40 °C/ 30 mmHg) was added via a glass pipet. The reaction solution was stirred for a further 18 h at rt and then quenched by the addition of NH4Cl (50 mg, 0.93 mmol) and MeOH (5 mL) and concentrated in vacuo. The crude reaction mixture was analyzed by 1H NMR to show ∼60% conversion by comparison with starting material 21 and oxidized product 23. (()-(R)-3-(((R)-1-Benzyl-6-oxopiperidin-3-yl)methyl)-4-(bis(trimethylsilyl)amino)-3,4-dihydropyrimidin-2(1H)-one and (()-(S)3-(((R)-1-Benzyl-6-oxopiperidin-3-yl)methyl)-4-(bis(trimethylsilyl)amino)-3,4-dihydropyrimidin-2(1H)-one (35). To a solution of NaHMDS (124 mg, 0.67 mmol) in THF (3 mL) at -78 °C was added a solution of 26a (100 mg, 0.34 mmol) in THF (3 mL, freshly distilled from sodium and benzoquinone) dropwise. The reaction solution was stirred at -78 °C for 2 h, quenched by the addition of saturated aq NH4Cl (0.5 mL), and further diluted with water (3 mL). The aqueous phase was extracted with CH2Cl2 (3  3 mL), and then the combined organic washings were dried (MgSO4) and concentrated in vacuo. Purification by flash chromatography [CH2Cl2/MeOH (49:1)] yielded adduct 35 (46 mg, 30%) as a 1:1 mixture of diastereomers and as a pale yellow oil: Rf 0.45 [CH2Cl2/MeOH (19:1)]; 1 H NMR (400 MHz, CDCl3) δH 7.37 (1H, m), 7.23-7.34 (5H, m), 5.96 (1H, m), 5.23 (1H, dd, J=19.0, 3.5 Hz), 4.79 (0.5H, d, J = 15.0 Hz), 4.60 (1H, m), 4.56 (1H, m), 4.42 (0.5H, d, J = 15.0 Hz), 3.53-3.66 (1H, m), 3.13-3.26 (1H, m), 2.94-3.09 (2H, m), 2.54-2.70 (1H, m), 2.36-2.50 (1H, m), 2.10-2.30 (1H, m), 1.84-1.94 (1H, m), 1.49-1.63 (1H, m), 0-0.25 (18H, br m); 13C NMR (101 MHz, CDCl3) δC 169.7, 169.5, 153.1, 152.9, 137.1, 136.9, 128.6, 128.5, 128.0, 128.0, 127.3, 127.3, 122.8, 122.8, 102.8, 69.1, 68.8, 50.8, 50.6, 50.4, 50.1, 44.1, 43.7, 32.7, 32.5, 31.5, 31.3, 25.5, 25.3, 2.5-3.1 (br), 2.4, 1.3. This material was subjected to NMR analysis immediately after isolation, and was found to be unstable, undergoing slow decomposition to regenerate 26a (as judged by 1H NMR) over a period of hours.

Acknowledgment. We thank Mr. Richard Hart and AstraZeneca for supporting the in situ IR experiments described here, and funding from EPSRC (to C.A.H. and P.M.D.) and the Rotary Foundation (to C.H.) is acknowledged. Supporting Information Available: Additional synthetic procedures for new compounds not described in the Experimental Section, characterization data and copies of 1H and 13C NMR spectra of new compounds, descriptions of NOE interactions for 2β, 17a, 17b, 17c and 22β, as well as expanded versions of the in situ IR data shown in Schemes 4 and 5. This material is available free of charge via the Internet at http:// pubs.acs.org.